Metallic nanoparticles draw intense scientific and practical interest due to their unique properties, which differ from those of bulk and atomic species. Such a difference is determined by peculiarity of electronic structure of the metal nanoparticles and extremely large surface area with a high percentage of surface atoms. Metal nanoparticles exhibit a drastic decrease in melting point compared to that of the bulk material, they are characterized by enhanced reactivity of the surface atoms, high electric conductivity and unique optical properties. Virtually, nanosized materials are well-known materials with novel properties and promising applications in electrochemistry, microelectronics, optical, electronic and magnetic devices and sensors, in new types of active and selective catalysts, as well as in biosensors. Creation of stable concentrated nanocolloids of metals with low resistivity offers new prospects in computer-defined direct-write noncontact technologies, such as ink-jet printing, for deposition of metallic structures on various substrates. Microfabrications of such structures by lithographic and electroless techniques are time-consuming and expensive processes, and there is a real industrial need for direct digital printing of conductive patterns. Suggestions based on jetting small droplets of molten metals onto the substrate have met several problems, such as difficulty of adhering droplets onto a substrate, oxidation of the liquid metal, and the difficulty of fabrication a droplet-ejection mechanism compatible with high temperatures. Direct patterning by ink-jet printing, in addition to the conventional graphic applications, was reported in the last decade for various applications, such as fabrication of transistors and organic light emitting diodes, polymer films, structural ceramics and biotechnology.
Conventional ink-jet inks may contain two types of colorants, dye or pigment, and are characterized by their main liquid, which is the vehicle for the ink. The main liquid may be water (water-based inks), or an organic solvent (solvent-based inks).
The dye or pigment-based inks differ with respect to the physical nature of the colorant. Pigment is a colored material that is insoluble in the liquid, while the dye is soluble in the liquid. Each system has drawbacks: pigments tend to aggregate, and therefore clog the nozzles in the orifice plate, or the narrow tubings in the printhead, thus preventing the jetting of the ink while printing. Dyes tend to dry, and form a crust on the orifice plate, thus causing failure in jetting and misdirection of jets.
It is clear that the terms “dye” or “pigment” are the general wordings for materials, which are soluble or insoluble, respectively, in the solvents comprising the ink. Therefore, metal nanoparticles may be considered, in this context, if introduced into ink, as pigments of metal, having a size in the nanometer range.
Conventional pigments in ink-jet inks contain particles in the size range of 100-400 nm. In theory, reducing the particle size to 50 nm or less should show improved image quality and improved printhead reliability when compared to inks containing significantly larger particles.
The majority of inks in ink-jet printers are water-based inks. The use of metal nanoparticles as pigments requires the elaboration of ink formulations containing stable concentrated aqueous metal colloid. The synthesis of stable colloidal systems with high metal concentration is a serious problem. A variety of substances have been used to stabilize silver colloids: amphiphilic nonionic polymers and polyelectrolytes, ionic and nonionic surfactants, polyphosphates, nitrilotriacetate, 3-aminopropyltrimethoxysilane, and CS2. Stable water-soluble silver nanoparticles were also obtained by reduction of silver ions in the presence of amino- and carboxilate-terminated poly(amido amine) dendrimers, and crown ethers. The preparations of stable silver colloids, having low metal concentrations are described in the literature, in procedures based on reduction of metal from solution. The metal concentrations in these procedures amount only to 10−2 M (about 0.1%) even in the presence of stabilizers (it is almost impossible to obtain a stable aqueous silver colloid with the metal concentrations higher then 10−3 M without an additional stabilizer, due to fast particle aggregation). The preparation of ink compositions having silver nanoparticle concentration of up to about 1.5 wt % (during the reaction step) is described in WO 03/038002.
The synthesis of concentrated silver nanoparticles is described in:
B. H. Ryu et al., Synthesis of highly concentrated silver nanoparticles, assisted polymeric dispersant, KEY ENGINEERING MATERIALS 264-268: 141-142 Part 1-3 2004;
Beyong-Hwan Ryu et al., Printability of the synthesized silver nano sol in micro-patterning of electrode on ITO glass, Asia display/IMID 04 Proceedings, pages 1-4;
Ivan Sondi et al., Preparation of highly concentrated stable dispersions of uniform silver nanoparticles, Journal of colloid and Interface Science, 260 (2003) 75-81;
Dan V. Goaia et al., Preparation of monodispersed metal particles, New J. Chem. 1998, pages 1203-1215.
Since ink-jet ink compositions contain, in addition to dyes or pigments, other additives, such as humectants, bactericides and fungicides and binders (polymeric additives, which improve the dye or pigment binding to substrate), the stabilizers should be compatible with these substances and should not change noticeably the physicochemical and rheological characteristics of inks (the most important characteristics are viscosity and surface tension).
Several methods of the metallic image generation with the use of ink-jet technology have been described.
One known method is based on an ink containing a reducing agent and receiving material containing the reducible silver compound (AgNO3 or silver di(2-ethylhexyl)-sulphosuccinate), and, on the contrary, an ink and a receiving support containing a silver compound and reducer, respectively. Heating the receiving support during or after the ink deposition resulted in an image formed by silver metal (U.S. Pat. No. 5,501,150 to Leenders, et al; U.S. Pat. No. 5,621,449 to Leenders, et al).
Another approach for the deposition of metallic structures is based on ink-jet printing of organometallic precursor dissolved in organic solvent with subsequent conversion of the precursor to metal at elevated temperatures (˜300° C.). To increase the metal (silver) loading of ink and to obtain higher decomposition rates, silver or other metal nanoparticles may be added to the ink along with the organometallic precursor. Near-bulk conductivity of printed silver films has been achieved with such compositions (Vest, R. W.; Tweedell, E. P.; Buchanan, R. C. Int. J. Hybrid Microelectron. 1983, 6, 261; Teng, K. F.; Vest, R. W. IEEE Trans. Indust. Electron. 1988, 35, 407; Teng, K. F.; Vest, R. W. IEEE Electron. Device Lett. 1988, 9, 591; Curtis, C.; Rivkin, T.; Miedaner, A.; Alleman, J.; Perkins, J.; Smith, L.; Ginley, D. Proc. of the NCPV Program Review Meeting. Lakewood, Colo., USA, 14-17 October 2001, p. 249).
Fuller et al. demonstrated inkjet printing with the use of colloidal inks containing 5-7 nm particles of gold and silver in an organic solvent, α-terpineol, in order to build electrically and mechanically functional metallic structures. When sintered, the resistivity of printed silver structures was found to be 3 μΩ·cm, about twice of that for bulk silver (Fuller, S. B.; Wilhelm, E. J.; Jacobson, J. M. J. Microelectromech. Syst. 2002, 11, 54).
The inventors have previously described the preparation of stabilized nanodispersions with silver concentration up to 1.5 wt %, at the reaction step which were shown to be suitable pigments for water-based ink-jet inks (WO 03/038002; Magdassi, S.; Bassa, A.; Vinetsky, Y.; Kamyshny, A. Chem. Mater. 2003, 15, 2208). The stabilizers used were ionic polymeric materials such as carboxymethyl cellulose (CMC) and polypyrrole (PPy), the silver nanoparticles size did not exceed 100 nm.
There is a widely recognized need and it will be highly advantageous to have a new method for obtaining aqueous-based dispersion of metal nanoparticles, preferably silver nanoparticles, which is simplified in production, which enables production of metal nanodispersion characterized by small diameter of the nanoparticles and high nanoparticle concentration and yet which is physically stable (i.e. does not undergo caking or agglomeration and can be easily redispersed if present as a sediment or a powder). Additionally it would be highly advantageous to have an aqueous based dispersion of metal nanoparticles with improved properties such as high electric conductivity when applied onto a substrate.